Mark E. Luther

The seasonal circulation of the upper ocean in the Bay of Bengal

Analysis of the results of a multilayer, adiabatic, numerical model of the upper Indian Ocean, driven by climatological monthly mean winds, shows that the simulated currents in the northeastern Indian Ocean are in general agreement with available observations and interpretations. The main features of the ocean currents include large anticyclonic flow in the Bay of Bengal surface waters during the northern hemisphere winter. This gyre decays into eddies in spring and then transitions into a weaker, cyclonic gyre by late summer. The western recirculation region of this flow is an intensified western boundary current which changes direction twice during the year. In the Andaman Sea, east of the Bay of Bengal, the oceanic flow changes direction twice during the year; it is cyclonic during the spring and early summer and anticyclonic the rest of the year. Flow in the equatorial region shows the North Equatorial Current (NEC) flowing west during winter. Further south is the eastward flowing Equatorial Counter Current (ECC) and the westward flowing South Equatorial Current. In summer, the NEC switches direction, joins the ECC, and forms the Indian Monsoon Current. Investigation of the second layer of the model (the upper 450 m of the ocean) shows that flow during much of the year is baroclinic (strong vertical shear). Model layer thickness reveals coastal Kelvin waves propagating along the coast, traveling the entire perimeter of the Andaman Sea and the Bay of Bengal. This wave excites westward propagating Rossby waves into the interior of the bay. Time series analysis of transport calculations yield significant peaks in the 20- to 30-day range and 50- to 60-day range which are not likely directly forced by the applied wind stress.

The Phytoplankton Bloom in the Northwestern Arabian Sea During the Southwest Monsoon of 1979

The biological variability of the northwestern Arabian Sea during the 1979 southwest monsoon has been investigated by the synthesis of satellite ocean color remote sensing with analysis of in situ hydrographic and meteorological data sets and the results of wind-driven modeling of upper ocean circulation. The phytoplankton bloom in the northwestern Arabian Sea peaked during August-September, extended from the Oman coast to about 65°E, and lagged the development of open-sea upwelling by at least 1 month. In total, the pigment distributions, hydrographic data, and model results all suggest that the bloom was driven by spatially distinct upward nutrient fluxes to the euphotic zone forced by the physical processes of coastal upwelling and offshore Ekman pumping. Coastal upwelling was evident from May through September, yielded the most extreme concentrations of phytoplankton biomass, and along the Arabian coast was limited to the continental shelf in the promotion of high concentrations of phytoplankton. Upward Ekman pumping to the northwest of the Somali Jet axis stimulated the development of a broad open-sea phytoplankton bloom oceanward of the Oman shelf. Vertical mixing during the 1979 southwest monsoon was apparently not a primary cause of the regional-scale phytoplankton bloom.

ENSO Influences on Seasonal Rainfall and River Discharge in Florida

Abstract Hemispherical and regional analyses of climatic patterns relating to El Nino–Southern Oscillation (ENSO) indicate strong responses in the southeastern United States, especially during the wintertime. Using Florida as an example, the authors focused on local-scale patterns within this region in order to examine the geographic variability of seasonal rainfall and river discharge as related to ENSO. Forty-eight years (1950–98) of precipitation and river discharge data in Florida were classified, using sea surface temperature anomaly data from the equatorial Pacific Ocean, as occurring during an El Nino (warm event), La Nina (cold event), or neither (neutral). Seasonal precipitation and streamflow both exhibited strong responses to ENSO as shown by their relationships to Nino-3.4 sea surface temperature anomalies. Florida does not respond as a uniform region to ENSO, particularly with respect to precipitation in the Panhandle and the southernmost areas of Florida. In particular, seasonal river discha...

A model of the seasonal circulation in the Arabian Sea forced by observed winds

Abstract Results of a numerical model of the wind driven seasonal circulation in the Arabian Sea are presented, with particular emphasis on the oceans response to the monsoon winds. The model equations are the fully nonlinear reduced gravity transport equations in spherical coordinates. The model resolution is 1/8° in the east-west direction and 1/4° in the north-south direction. The model basin geometry corresponds as closely as possible to that of the north-west Indian Ocean from 40°E to 73°E and from 10°S to 25°N, and includes the gulfs of Aden and Oman, and the islands of Socotra and the Seychelles. The southern boundary and a portion of the eastern boundary, from the equator to 6°S, representing the opening between the Maldives and the Chagos Archipelago, are open boundaries. At other boundaries, the no-slip condition is applied. The wind stress data used to force the model comes from the NOAA National Climate Centers TD-9757 Global Marine Sums data, which consists of monthly mean winds compiled on 1° squares from over 60 years of ship observations. These data are interpolated in time using the mean and first five Fourier harmonics at each point, and then interpolated linearly to the model grid. The model equations are integrated in time using centered finite differences in time and space (a leap-frog scheme), with lateral friction treated by a Dufort-Frankel scheme. After a one year spin up, the model settles into a regular periodic seasonal cycle, even though the solution to the model equations is locally highly nonlinear, with large nonlinear eddies developing in the same location at the same time of year from one year to the next. The development of the model Somali Current system with the onset of the (northern hemisphere) summer monsoon is consistent with the available observations in the region. The model reproduces many of the observed features in this region, such as the two-gyre circulation pattern, and the timing and movement of these features corresponds well with their real world counterparts. The model also shows an eastward jet forming in late June to early July at 13°N, just to the east of Socotra. This jet is fed by flow coming out of the great whirl. The break down of the two-gyre pattern occurs in mid to late August, when the southern gyre breaks up into several smaller eddies, the northern-most of which coalesces with the great whirl. Numerous small cyclonic eddies develop along the Arabian coast, from the Gulf of Oman into the Gulf of Aden, in early to mid August, and persist well into the winter monsoon. The model shows that it is possible to simulate very complicated flows, if one has sufficient wind data, using fairly simple models with a realistic basin geometry.

ENSO impacts on salinity in Tampa Bay, Florida

Estuarine salinity distributions reflect a dynamic balance between the processes that control estuarine circulation. At seasonal and longer time scales, freshwater inputs into estuaries represent the primary control on salinity distribution and estuarine circulation. El Niño-Southern Oscillation (ENSO) conditions influence seasonal rainfall and stream discharge patterns in the Tampa Bay, Florida region. The resulting variability in freshwater input to Tampa Bay influences its seasonal salinity distribution. During El Niño events, ENSO sea surface temperature anomalies (SSTAs) are significantly and inversely correlated with salinity in the bay during winter and spring. These patterns reflect the elevated rainfall over the drainage basin and the resulting elevated stream discharge and runoff, which depress salinity levels. Spatially, the correlations are strongest at the head of the bay, especially in bay sections with long residence times. During La Niña conditions, significant inverse correlations between ENSO SSTAs and salinity occur during spring. Dry conditions and depressed stream discharge characterize La Niña winters and springs, and the higher salinity levels during La Niña springs reflect the lower freshwater input levels.

The 26-day oscillation observed in the satellite sea surface temperature measurements in the equatorial western Indian Ocean

A 26-day oscillation in sea surface temperature (SST) data is observed in the western Indian Ocean, from 52° to 60°E and in the vicinity of the equator. The SST data used in this study are obtained from the NOAA 9 satellite and are for the years 1987 and 1988. This fluctuation of SST at a period near 26 days is found to be antisymmetric about the equator and is trapped within the equatorial waveguide (equator ±6°). The variance associated with this oscillation has a maximum located at about 3° latitude; furthermore, the variance decreases at a faster rate toward the equator than poleward. These characteristics are consistent with the latitudinal structure for the mixed Rossby-gravity (or Yanai) waves as predicted from linear wave theory. The temporal variation of this 26-day oscillation is most energetic during the summer season (July to September), with maximum values of 0.4°C and 0.8°C found during August of 1987 and 1988 respectively. This observation agrees with the temporal variation of Yanai waves inferred from drifting buoy observations and numerical studies of the Indian Ocean. Thus we conclude that the Yanai wave is responsible for the 26-day fluctuation observed in the SST data in this region.

Mixed Instabilities in the Gulf Stream over the Continental Slope

Abstract A numerical model study is presented of the unstable normal modes of oscillation of a boundary current. The model background current approximates the Gulf Stream south of Cape Hatteras, North Carolina. Both vertical and horizontal shear in current velocity and a sloping bottom topography are included. The study seeks small amplitude, alongshore propagating perturbations with real frequency and complex alongshore wavenumber. A nonzero imaginary part of the wavenumber ensures that the wave amplitude either grows or decays in the alongshore direction. The first four eigenmodes are identified and their dispersion relations are investigated. Higher order modes are not resolved by the model. The dispersion surfaces (eigenvalues of frequency as a function of complex wavenumber) appear to bifurcate with increasing values of real wave number. Observations in the Gulf Stream south of Cape Hatters have revealed a persistent wave-like meander pattern in the Stream with a period of 7–8 days. This wave form pr...

Determining the Effects of El Nino-Southern Oscillation Events on Coastal Water Quality

The importance of the El Niño-Southern Oscillation (ENSO) on regional-scale climate variability is well recognized although the associated effects on local weather patterns are poorly understood. Little work has addressed the ancillary impacts of climate variability at the community level, which require analysis at a local scale. In coastal communities water quality and public health effects are of particular interest. Here we describe the historical influence of ENSO events on coastal water quality in Tampa Bay, Florida (USA) as a test case. Using approximate randomized statistics, we show significant ENSO influences on water quality particularly during winter months, with significantly greater fecal pollution levels during strong El Niño winters and significantly lower levels during strong La Niña winters as compared to neutral conditions. Similar significant patterns were also noted for El Niño and La Niña fall periods. The success of the analysis demonstrates the feasibility of assessing local effects associated with large-scale climate variability. It also highlights the possibility of using ENSO forecasts to predict periods of poor coastal water quality in urban region which local agencies may use to make appropriate prepations.

Very high-frequency radar mapping of surface currents

An ocean surface current radar (OSCR) in the very high frequency (VHF) mode was deployed in South Florida Ocean Measurement Center (SFOMC) during the summer of 1999. During this period, a 29-d continuous time series of vector surface currents was acquired starting on 9 July 1999 and ending 7 August 1999. Over a 20-min sample interval, the VHF radar mapped coastal ocean currents over a 7.5 km /spl times/ 8 km domain with a horizontal resolution of 250 m at 700 grid points. A total of 2078 snapshots of the two-dimensional current vectors were acquired during this time series and of these samples, only 69 samples (3.3%) were missing from the time series. During this period, complex surface circulation patterns were observed that included coherent, submesoscale vortices with diameters of 2 to 3 km inshore of the Florida Current. Comparisons to subsurface measurements from moored and ship-board acoustic Doppler current profiles revealed regression slopes of close to unity with biases ranging from 4 to 8 cm s/sup -1/ between surface and subsurface measurements at 3 to 4 m beneath the surface. Correlation coefficients were 0.8 or above with phases of - 10 to - 20/spl deg/ suggestive of an anticyclonic veering of current with depth relative to the surface current. The radar-derived surface current field provided spatial context for an observational network using mooring-, ship- and autonomous underwater vehicle-sensor packages that were deployed at the SFOMC.

Rapid changes in the seasonal sea level cycle along the US Gulf coast from the late 20th century

Temporal variations of the seasonal sea level harmonics throughout the 20th and early 21st century along the United States Gulf coast are investigated. A significant amplification of the annual sea level cycle from the 1990s onward is found, with both lower winter and higher summer sea levels in the eastern Gulf. Ancillary data are used to build a set of multiple regression models to explore the mechanisms driving the decadal variability and recent increase in the annual cycle. The results suggest that changes in the air surface temperature toward warmer summers and colder winters and changes in mean sea level pressure explain most of the amplitude increase. The changes in the seasonal sea level cycle are shown to have almost doubled the risk of hurricane induced flooding associated with sea level rise since the 1990s for the eastern and north-eastern Gulf of Mexico coastlines.